Colorless digital primer for digital printing

ABSTRACT

A colorless digital primer for digital printing includes a colorless paste. The colorless paste includes a non-polar carrier fluid, and a resin swollen in the non-polar carrier fluid. The resin is present in the paste in an amount ranging from about 20% (w/w) to about 50% (w/w). A solid polar compound is dispersed in the resin. The solid polar compound is selected from the group consisting of a cellulose microcrystalline powder, dextrin, maltose monohydrate, polyacrylic acid, polyvinyl alcohol, a styrene maleic anhydride copolymer, a bismaleimide oligomer, sucrose, sucrose octaacetate, sucrose benzoate, and combinations thereof. The solid polar compound is present in an amount up to 60 wt. % of solids in the colorless paste.

BACKGROUND

The global print market is in the process of transforming from analogprinting to digital printing. Inkjet printing and electrophotographicprinting are two examples of digital printing techniques. Liquidelectrophotographic (LEP) printing is an example of electrophotographicprinting that utilizes a liquid toner-based ink as opposed to a drytoner. LEP printing combines the electrostatic image creation of laserprinting with the blanket image transfer technology of offsetlithography. In one example of LEP printing, an ink image iselectrostatically transferred from a photo imaging plate (i.e.,photoconductor, photoconductive drum, photoreceptor, etc.) to anintermediate drum or roller, and then is transferred to a desirablemedium.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of examples of the present disclosure willbecome apparent by reference to the following detailed description andthe drawings, in which like reference numerals correspond to similar,though perhaps not identical, components. For the sake of brevity,reference numerals or features having a previously described functionmay or may not be described in connection with other drawings in whichthey appear.

FIG. 1 is a flow diagram illustrating an example of a method of makingan example of a colorless digital primer;

FIG. 2 is a schematic diagram of a liquid electrophotographic printingsystem for printing examples of the colorless digital primer disclosedherein;

FIG. 3 is a scanning electron microscope (SEM) image of an example ofthe printed colorless primer including maltose monohydrate dispersed ina colorless paste of ElectroInk 4.5 based on ethylene methacrylic acidcopolymers and ethylene acrylic acid copolymers; and

FIGS. 4A and 4B are black and white representations of color photographsof LEP prints exposed to a peeling test at different times afterprinting, where the respective LEP prints included i) a cyan LEP inkalone (FIG. 4A) and ii) the cyan ink printed on an example of thecolorless digital primer according to the present disclosure (FIG. 4B).

DETAILED DESCRIPTION

Examples of the colorless digital primer disclosed herein may be applieddirectly on a substrate surface in order to function as an undercoat forany subsequently applied LEP ink(s). The colorless digital primer may beused to prepare a difficult substrate (e.g., polymeric substrate, somepaper substrates, etc.) for the subsequent application of the LEPink(s). The colorless primer improves the durability of the print, forexample, against mechanical wear (e.g., peeling). The improveddurability may be due, at least in part, to the presence of solid polarcompounds and a relatively small amount of a non-polar carrier fluid inthe colorless primer. The solid polar compounds reduce the amount ofresin that swells with non-polar carrier fluid, in turn reducing theamount of non-polar carrier fluid in, and to be removed from, theprinted primer layer. It is believed that the non-polar carrier fluidpresent on the substrate decreases the interaction between the polargroups on the substrate and the polar groups in the LEP ink(s) printedon the substrate. Reducing the amount of carrier fluid in the printedprimer increases the polar interaction between the substrate and theprinted primer. Furthermore, the solid polar compounds in the printedprimer layer interact with the polar groups of the LEP ink (e.g., acidicgroups on the ink resin) and with the polar groups on the substrate. Asa result, the effect of the non-polar carrier fluid is diminished, andoverall adhesion is improved.

The colorless digital primer disclosed herein includes a colorless pasteand a solid polar compound dispersed in the resin of the colorlesspaste. The colorless primer does not include any colorant materials(e.g., pigments, dyes, etc.). In the examples disclosed herein, thecolorless primer may also include a charge adjuvant and a chargedirector, which impart charges to the primer. The various components ofexamples of the colorless primer will be described further in referenceto FIG. 1, which illustrates an example of a method 100 for making thecolorless digital primer.

Referring now to step 102 of FIG. 1, an example of the method 100includes selecting a solid polar compound that is suitable for thecolorless digital primer. It has been found that a select few solidpolar compounds are suitable for use in the colorless digital primerdisclosed herein. These solid polar compounds are selected from thefollowing group: a cellulose microcrystalline powder, dextrin, maltosemonohydrate, polyacrylic acid, polyvinyl alcohol, a styrene maleicanhydride copolymer, a bismaleimide oligomer, sucrose, sucroseoctaacetate, sucrose benzoate, and combinations thereof. Examples ofcommercially available styrene maleic anhydride copolymers include thosefrom Sartomer Co. USA, LLC, such as SMA® 4000I and SMA® 1000I (i.e.,styrene and dimethyl amino propylamine (DMAPA) maleimides) and SMA®1000P (styrene maleic anhydride resin, cumene end-capped). A suitableexample of a bismaleimide oligomer is bis-stearamide.

All of these solid polar compounds contains polar atoms, such as oxygen,nitrogen, etc., that prevent the solid compounds from dissolving or evenswelling in the non-polar carrier fluid. As such, the solid polarcompounds do not interact with the non-polar carrier fluid, but ratherare dispersed in the resin of the colorless paste.

The solid polar compounds are also grindable (i.e., capable ofundergoing particle size reduction when exposed to a grinding process).While all of the listed solid polar compounds are grindable, some may bemore easily ground than others.

A desirable particle size for the solid polar compounds in the colorlessprimer is on the nanoscale, and ranges from about 30 nm to about 300 nm.In another example, the particle size of the solid polar compoundsranges from about 70 nm to about 130 nm. In still other examples, thedesirable particle size for the solid polar compounds in the colorlessprimer ranges from about 50 nm to about 150 nm. An example of theprinted colorless primer having solid polar compounds on the nanoscaleis shown in FIG. 3. More particularly, FIG. 3 illustrates a SEM of anexample of the colorless primer (including maltose monohydrate andtransparent ElectroInk 4.5) disclosed herein after it is printed. Asdepicted, the particle size of the solid polar compounds is as low asabout 77 nm and as high as about 104 nm.

The desired particle size may be achieved during the grinding of thecompounds with the colorless paste (reference numeral 106) or may beachieved in a grinding process (reference numeral 108) that is performedprior to adding the solid polar compounds to the colorless paste.

Referring now to step 104 in FIG. 1, the method 100 includes adding theselected solid polar compound to the colorless paste. The colorlesspaste is a dispersion of a resin swollen in a non-polar carrier fluid.

The non-polar carrier fluid of the colorless paste is an electricalinsulator, having a resistivity of at least about 10⁹ ohm-cm. Thenon-polar carrier fluid may be a hydrocarbon, examples of which includeisoparaffinic hydrocarbons, paraffinic hydrocarbons, aliphatichydrocarbons (e.g., an isomerized aliphatic hydrocarbon, a branchedchain aliphatic hydrocarbon, etc.), aromatic hydrocarbons, de-aromatizedhydrocarbons, halogenated hydrocarbons, cyclic hydrocarbons,functionalized hydrocarbons, and combinations thereof.

Some examples of the non-polar carrier fluid include ISOPAR® G, ISOPAR®H, ISOPAR® K, ISOPAR® L, ISOPAR® M, ISOPAR® V, NORPAR® 12, NORPAR® 13,NORPAR® 15, EXXOL® D40, EXXOL® D80, EXXOL® D100, EXXOL® D130, and EXXOL®D140, all of which are available from Exxon-Mobil Corp., Houston, Tex.Further examples of the non-polar carrier fluid include Electron,Positron, New II, and Purogen HF (all of which are available fromEcolink Co., Tucker, Ga.). Still further examples of the non-polarcarrier fluid include TECLEN® N-16, TECLEN® N-20, TECLEN® N-22, NISSEKINAPHTHESOL® L, NISSEKI NAPHTHESOL® M, NISSEKI NAPHTHESOL® H, Solvent H,Solvent L, Solvent M, NISSEKI ISOSOL® 300, NISSEKI ISOSOL® 400, AF-4,AF-5, AF-6, and AF-7 (all of which are available from Nippon Oil Corp.,Tokyo, JP); IP Solvent 1620 and IP Solvent 2028 (both of which areavailable from Idemitsu Petrochemical Co., Ltd., Tokyo, JP); and AMSCO®OMS and AMSCO® 460 (both of which are available from American MineralSpirits Co., Los Angeles, Calif.).

The resin in the colorless paste may be any solid polymer that is ableto swell in the selected non-polar carrier fluid. By swelling, it ismeant that the resin is capable of increasing in size as a result ofaccumulation of the non-polar carrier fluid. The resin selected is alsoable to emit the carrier fluid when phase separation is initiated (e.g.,when the swollen resin is exposed to heat at a temperature ranging fromabout 50° C. to about 80° C., for example, during printing). Examples ofthe swellable resin include ethylene acrylic acid copolymers and/orethylene methacrylic acid copolymers. Both ethylene acrylic acidcopolymers and ethylene methacrylic acid copolymers are commerciallyavailable under the tradename NUCREL® from E. I. du Pont de Nemours andCompany, Wilmington, Del. The swelling of these types of resins may bedue, at least in part, to the molecular structure similarity between theethylene-based resin(s) and the non-polar carrier fluid. It is to beunderstood that any other homopolymer or copolymer that is capable ofswelling in the non-polar carrier fluid and is also capable of releasingthe non-polar carrier fluid when exposed to suitable heat conditions mayalso be used.

The resin(s) may be mixed with the non-polar carrier fluid, and themixture may be exposed to heating while stirring to induce swelling. Thetemperatures used during mixing may vary depending upon the type ofresin. In an example, the temperature ranges from about 120° C. to about200° C. After heating, the resin(s) in the non-polar carrier fluid maybe cooled while stirring is continued. When the resin(s) is/are swollenin the non-polar carrier fluid, the resulting dispersion is thecolorless paste. In an example, the resin(s) makes up from about 20%(w/w) to about 50% (w/w) of the colorless paste. The remainder of thecolorless paste includes the non-polar carrier fluid, some of which ispresent in the swollen resin and some of which is present as freecarrier (i.e., liquid that is not swollen in the resin(s)).

The colorless paste may include other additives, such as derivatives ofpolytetrafluoroethylene (PTFE) and polyethylene wax. These additives donot swell in the non-polar carrier fluid. These additives may beincluded in the colorless paste in an amount ranging from about 1 wt. %to about 20 wt. % of the total wt. %.

As mentioned above, step 104 in FIG. 1 illustrates adding the solidpolar compound to the colorless paste. The solid polar compound may beadded in any amount up to 60 wt. % of total solids in the colorlesspaste. In an example, the amount of the solid polar compound that isadded ranges from about 10 wt. % of total solids to about 60 wt. % oftotal solids in the colorless paste. The inclusion of the solid polarcompounds reduces the amount of the resin that becomes swollen with thenon-polar carrier fluid.

Prior to adding the solid polar compounds to the colorless paste, thesolid polar compounds may be pre-ground (reference numeral 108) in orderto achieve the desired particle size. Whether or not the solid polarcompounds are pre-ground to the desirable size, after the solid polarcompounds are added to the colorless paste, the mixture is ground, asshown at reference numeral 106. This process disperses the solid polarcompounds throughout the resin of the colorless paste, and, as notedabove, may also (further) reduce the particle size of the solid polarcompounds.

The grinding process shown at reference numeral 106 involves milling themixture of the colorless paste and the solid polar compounds. Millingmay be accomplished in any suitable mixing apparatus, such as anattritor.

Grinding/milling is accomplished using suitable conditions (e.g., speed,temperature, etc.) for a suitable time to disperse the solid polarcompounds in the resin of the colorless paste. In an example, the speedmay be about 250 rpm and the temperature may range from about 25° C. toabout 40° C. In one example, grinding may be accomplished for about 12hours to about 48 hours. This process causes the solid polar compoundsto disperse throughout the resin, thereby occupying area(s) within theresin that may otherwise become swollen with the non-polar carrierfluid. In some instances, this process also causes the solid polarcompounds to deagglomerate into discrete particles. For instance, duringmilling, the solid polar compound agglomerates may break up intodiscrete particles, which have an average particle size ranging fromabout 30 nm to about 300 nm.

During the grinding/milling process, the particle size may be monitored,e.g., via dynamic light scattering (DLS). After the grinding/millingprocess, the particle size may be determined using a Malvern particlesize analyzer. The particle size of the solid polar compounds in theapplied film/layer may also be determined using scanning electronmicroscopy (SEM) (see, for example, FIG. 3).

As shown at reference numeral 110, a charge adjuvant may be added to themixture during the grinding process. This causes the charge adjuvant tobecome part of the swollen resin(s), by virtue of chemical bonding orphysical association.

In one example, the charge adjuvant includes a base moiety that binds tothe acidic groups of the resin(s). The charge adjuvant also provides amolecular structure to trap charge director molecules around theresin(s). For example, the charge adjuvant may include an electrondonating moiety (e.g., a Lewis base or an organo-Lewis base) thatinteracts with a negative charge director to enhance the negativecharge. In this example, the donor charge adjuvant is believed toenhance the binding and/or activation of the charge director and thusincrease the negative conductivity of the colorless digital primer.

Examples of the donor charge adjuvant include metallic soaps containinga metal, such as Al, Zn, Ca, Mg, Y, other metals, and combinationsthereof, and a ligand, such as stearate, oleate, palmitate, otherligands, and combinations thereof. Examples of metallic soaps includealuminum tristearate, aluminum distearate, polyoxo aluminum stearate(POAS), polyoxo aluminum palmitate, oxo-aluminum acrylates, Y(III)stearate, or any other metallic salt whose leaving group is capable ofdissolving in the non-polar carrier fluid.

In another example, the charge adjuvant includes an acidic moiety thatbinds to the basic groups of the resin(s). The charge adjuvant alsoprovides a molecular structure to trap charge director molecules aroundthe resin(s) and/or solid polar compound(s). For example, the chargeadjuvant may include an electron accepting moiety (e.g., a Lewis acid)that interacts with a positive charge director to enhance the positivecharge. In this example, the acceptor charge adjuvant is believed toenhance the binding and/or activation of the charge director and thusincrease the positive conductivity of the colorless digital primer.

Examples of the acceptor charge adjuvant have the formulaX_(n)(R^(a))(R^(b)) or X_(n)(R^(a))(R^(b))(COOH) where X is F, Cl, Br,NO₂ or CN; R^(a) is a substituted or unsubstituted alkyl group; R^(b) isSb, P, Ti, Sn, B, Al, Zn, or an aromatic group; and n is 1, 2, 3, 4 or5. One example of the acceptor charge adjuvant has the formulaCl_(n)(R^(a))(R^(b))(COOH), where R^(a) is a substituted orunsubstituted alkyl group having 3, 4 or 5 carbon atoms, R^(b) is abenzene or a phenol group, and n is 1 or 2. Another example of theacceptor charge adjuvant is a chlorobenzene based compound, such asCl(R^(a))(R^(b))(COOH), where R^(a) is an alkyl group having 5 carbonatoms and R^(b) is a benzene group. Still another example of theacceptor charge adjuvant has Cl₂(R^(a))(R^(b))(COOH), where R^(a) is analkyl group having 4 carbon atoms and R^(b) is a phenol group. Stillother examples this charge adjuvant include chloro-phenyl carboxylicacid, 2-(4-chlorophenyl)-3-methylbutyric acid, and4-(2,4-dichlorophenoxy)butyric acid.

The term “alkyl” as used in the examples of the acceptor charge adjuvantmeans a branched, unbranched or cyclic saturated hydrocarbon group,which may contain from 1 to 20 carbon atoms. Alkyls include, forexample, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl,octyl, and decyl, as well as cycloalkyl groups, such as cyclopentyl, andcyclohexyl. The alkyl may be a lower alkyl group, which includes from 1to 8 carbon atoms. In some examples, R^(a) is a substituted alkyl groupor a heteroalkyl alkyl group. As used herein, the term “substitutedalkyl” means an alkyl substituted with one or more substituent groups;and the term “heteroalkyl” means an alkyl in which at least one carbonatom is replaced with a heteroatom.

A suitable amount of the charge adjuvant ranges from about 0.05 wt % toabout 8 wt % of the total solids present in the colorless paste.

During grinding, it is to be understood that grinding aids may also beadded.

As shown at reference numeral 112, the colorless paste (which is in aconcentrated form having a solids content ranging from 20% (w/w) to lessthan 100% (w/w)) is diluted so that the solids content ranges from 1%(w/w) to 5% (w/w). In an example, the ground mixture is supplied to amachine (e.g., an ink tank of an LEP printer) in the concentrated form.Additional non-polar carrier fluid is added to dilute the mixture to thedesired solids content.

Referring now to reference numeral 114, a charge director is then addedto the diluted mixture of the colorless paste, the solid polarcompounds, and the charge adjuvant to form the colorless digital primer.Generally, charge director(s) are components that induce and/or increasethe charge of liquid electrophotographic inks. In the examples disclosedherein, the charge directors can induce and/or increase the charge ofthe colorless primer. It is believed that the charge director canphysically associate with the charge adjuvant (which is included(dispersed) in LEP ink particles or, in the examples disclosed herein,the swollen resin including the solid polar compounds). The selectedcharge director(s) may form a micelle (or a reverse micelle) structure.

As will be discussed further below, LEP printers include aphotoconductive surface which may be charged either positively ornegatively. The charge director imparts a charge to the LEP ink and/orcolorless primer which is identical to the charge of the photoconductivesurface. For example, if the photoconductive surface is chargednegatively, then a negative charge director may be used, and if thephotoconductive surface is positively charged, then a positive chargedirector may be used.

Examples of negative charge directors include organic multi-valent metalsurfactant salts. These organic salts are soluble in the non-polarcarrier fluid, for example, at room temperature. In one example, theorganic multi-valent metal salt includes polyvalent metal ions, andorganic anions as the counterion. Examples of suitable metal ionsinclude Ba(II), Ca(II), Mn(II), Zn(II), Zr(IV), Cu(II), Al(III),Cr(III), Fe(II), Fe(III), Sb(III), Bi(III), Co(II), La(III), Pb(II),Mg(II), Mo(III), Ni(II), Ag(I), Sr(II), Sn(IV), V(V), Y(III), Ta(V), andTi(IV). Examples of suitable organic anions include carboxylates orsulfonates derived from aliphatic or aromatic carboxylic or sulfonicacids. Other negative charge directors include polyisobutylenesuccinimide polyamines (an example of which includes OLOA®1200,available from Chevron Oronite). Still other examples of the negativecharge director include metal salts of fatty acids (e.g., calciumpalmitate); metal salts of sulfo-succinates; metal salts ofoxyphosphates; metal salts of alkyl-benzenesulfonic acid; metal salts ofaromatic carboxylic acids or sulfonic acids; metal salts of naphthenicacid (e.g., barium petronate); polyoxyethylated alkylamines; lecithin;polyvinyl-pyrrolidone; and/or organic acid esters of polyvalentalcohols. While several examples are provided, it is to be understoodthat other negative charge directors may also be utilized in theexamples disclosed herein.

Examples of positive charge directors include the organic multi-valentmetal surfactant salts and the polyisobutylene succinimide polyaminespreviously discussed. As specific examples, the positive charge directormay be a transition metal salt of a fatty acid, such as aluminumstearate, or a transition metal salt of naphthenic acid, such as cobaltnaphthanate. Still other specific examples of positive charge directorsinclude manganese naphthenate, manganese octoate, zirconium octoate,cobalt octoate, iron naphthenate, magnesium octoate,titanium(IV)2-ethyl-1,3 hexanediolate, titanium(IV)-2-ethylhexyloxide,zirconium(IV)-ter-butoxide, tantalum(V)-butoxide, poly-oxo-aluminumtristearate, zinc naphthenate, barium distearate and calcium stearate.In one example, the positive charge director is zirconium(IV) octoate or2-ethyl hexanoate. In another example, the positive charge director iszirconium(IV) 2-ethyl hexanoate (i.e., ZZ11). While several examples areprovided, it is to be understood that other positive charge directorsmay also be utilized in the examples disclosed herein.

A suitable amount of the charge director depends on the type of resin(s)and/or solid polar compound(s). In an example, the amount of chargedirector added to the diluted colorless primer ranges from about 10 mgper g of total solids to about 150 mg per g of total solids (from about1 wt. % to about 15 wt. % of the total solids present in the dilutedcolorless primer).

Once the colorless digital primer is made, it may be applied (digitallyand in a selective manner) as a film/layer directly on a substratesurface so that it acts as a primer for any subsequently applied LEPink(s). Examples of the LEP ink(s) include any commercially availableLEP ink (e.g., ElectroInk available from HP Indigo), and examples of thesubstrate include coated and uncoated papers. Examples include UPMFinesse (135 g/m²) from UPM Kymi Paper and Pulp Mill, Multifine 130g/m², which is an uncoated paper from StoraEnso, or polymeric(bi-oriented polypropylene) BOPP 38 micron corona treated paper.

The colorless primer may be applied/printed using any liquidelectrophotographic (LEP) digital printing press. A schematicillustration of an LEP digital printing press is shown in FIG. 2. Asillustrated, the LEP digital printing press 10 includes aphotoconductive drum 12 (including the previously mentionedphotoconductive surface), a laser source 14, a binary ink development(BID) unit 16, a blanket drum 18, and an impression cylinder 20. Whilenot shown, it is to be understood that the colorless digital primer (inthe concentrated form, i.e., prior to dilution) may be stored in an inktank (not shown), where the previously described dilution and chargingsteps may take place.

When printing the colorless digital primer disclosed herein, a latentimage is formed on the photoconductive drum 12 using the laser source14. In an example, it may be desirable to apply the colorless digitalprimer on the entire surface of the medium 22, and thus the latent imagemay be the shape of the medium 22 used. In another example, it may bedesirable to apply the colorless digital primer on only those portionsof the medium 22 that will ultimately have LEP ink applied thereon, andthus the latent image may be the desirable shape for the final printedimage. The laser source 14 selectively discharges the photoconductivedrum 12 to generate the latent image.

In an example, the BID unit 16 applies the colorless digital primer fromthe ink tank (not shown), and the colorless digital primer accumulateson a developer roller (which is part of the BID unit). Moreparticularly, the developer roller accumulates the colorless digitalprimer solids to form a charged paste-like layer. This chargedpaste-like layer is compacted, for example, by a squeegee. The chargedpaste-like layer on the developer roller of the BID unit 16 supplies thecolorless digital primer to the photoconductive drum 12, where thecharged paste-like layer accumulates on the imaged areas (i.e., thelatent image) alone of the photoconductive drum 12. In other words, thecharged colorless digital primer is attracted to the discharged area(s),while the charged area(s) of the photoconductive drum 12 repel thecharged colorless digital primer (creating a clean background). Any ofthe paste-like layer remaining on the developer roller after transfer tothe photoconductive drum 12 may be removed, for example, by a cleaningroller.

The paste-like layer (in the form of the latent image) on thephotoconductive drum 12 is then transferred to the blanket drum 18. Amedium 22 is directed between the blanket drum 18 and the impressioncylinder 20, and the paste-like layer on the blanket drum 18 istransferred to the medium 22 to form the desired primer film/layerdirectly on the surface of the medium 22.

In the example printing method described above, it is to be understoodthat LEP ink(s) may be printed in a similar manner after the applicationof the colorless digital primer layer.

In another example printing method, the LEP ink(s) and the colorlessdigital primer are applied to the medium 22 at the same time. In thisexample, the LEP ink and the colorless digital primer are accumulatedtogether on the blanket drum 18 and are transferred to the medium 22together. In this example, since the colorless digital primer is anunderlayer, it is accumulated on the blanket drum 18 above theaccumulated LEP ink so that when transferred to the medium 22, thecolorless digital primer layer serves as the primer or underlayer.

Any number of layers of the colorless digital primer may be printed. Itis desirable that the colorless digital primer remain transparent afterits application. In an example, one thin layer (e.g., about 1 μm thick)of the colorless digital primer is transparent. Printing more than fourof these thin layers may introduce some level of opaqueness. In order toavoid this, the refractive index of the colorless paste and the solidpolar compounds may be matched when preparing the colorless digitalprimer. By matching, it is meant that the refractive index of each ofthe colorless paste and the solid polar compounds is within the range of1.03 to 2.05. In an example, the refractive index of the colorless pasteis 1.53 and the refractive index of the solid polar compounds is 1.53±1.When the refractive indices are matched, the printed colorless digitalprimer remains transparent, even when multiple layers are printed.

To further illustrate the present disclosure, an example is givenherein. It is to be understood that this example is provided forillustrative purposes and is not to be construed as limiting the scopeof the present disclosure.

EXAMPLE

An example of the colorless digital primer disclosed herein wasprepared. The colorless digital primer included 38.5 wt. % of HPElectroInk 4.5 colorless paste (based on ethylene methacrylic acidcopolymers and ethylene acrylic acid copolymers), 60 wt. % ofpolyacrylic acid, and 1.5 wt. % aluminum stearate. These components wereground in an attritor at 25° C. for 24 hours. Using an HP Indigo 5000series printing system, the ground mixture was diluted with additionalnon-polar carrier fluid to a solids content ranging from about 2% (w/w)to about 3% (w/w). The colorless primer was charged by adding a chargedirector (Imaging Agent from HP).

A bi-oriented polypropylene (BOPP) 38 μm film was treated by corona (400W), cut and glued on top of a paper substrate (Condat digital gloss 135gr. from Condat, France). This medium was introduced into the HP indigo5000 printing system. For the sample, the colorless digital primer ofthis Example was printed on half of the medium, and then a cyan inklayer was printed thereon. For the comparative sample, the cyan inklayer alone was printed on the other half of the medium that did notinclude the colorless primer.

Peeling tests were performed (according to FINAT test method FTM 21 TestMethod No. 21) by adhering tape (3M SCOTCH® Magic Tape #810) to thesurface of the comparative image and the example image using a 2 kgrubber coated roller. The tape was then removed from the respectiveimages at 20 minutes after printing and at 60 minutes after printing.FIGS. 4A and 4B are black and white representations of color photographsof the test results. As depicted, the ink was peeled off from thenon-primed medium (FIG. 4A) while the ink remained in place on the sideof the medium primed with the colorless primer (FIG. 4B).

This Example demonstrates that a BOPP medium primed with a layer of thecolorless digital primer prior to ink application improves adhesionsignificantly.

While the results are not shown, similar peeling test results wereobtained when other difficult paper substrates, such as UPM Finesse 135gr./m2 from UPM, Kymi Paper and Pulp Mill, and Multifine 130 gr./m2 fromStoraEnso, were tested in a similar manner (i.e., ½ primed with a secondcolorless digital primer and the other ½ unprimed). As noted, a secondcolorless digital primer was used to test the UPM Finesse and Multifinesubstrates. The second colorless digital primer included 58.5 wt. % ofHP ElectroInk 4.5 colorless paste, 40 wt. % of maltose monohydrate orpolyacrylic acid, and 1.5 wt. % aluminum stearate. These components wereground in an attritor at 25° C. for 24 hours. Using an HP Indigo 5000series printing system, the ground mixture was diluted with additionalnon-polar carrier fluid to a solids content ranging from about 2% (w/w)to about 3% (w/w). The second colorless digital primer was charged byadding a charge director (Imaging Agent from HP).

For these additional samples, the second colorless digital primer wasprinted on half of the UPM Finesse and Multifine paper substrates, andthen a cyan ink layer was printed thereon. For the additionalcomparative samples, the cyan ink layer alone was printed on the otherhalf of the UPM Finesse and Multifine paper substrates that did notinclude the second colorless digital primer.

For the peeling tests involving the additional samples and theadditional comparative samples (i.e,. those made with the secondcolorless digital primer and the UPM Finesse and Multifine papersubstrates, tape (3M SCOTCH® Magic Tape #230) was adhered to the surfaceof the additional samples and comparative samples using a 1 kg rubbercoated roller. The tape was then removed from the additional samples andthe additional comparative samples at 10 minutes after printing. Both ofthe additional comparative samples exhibited peeling, and both of theadditional samples (including the cyan ink layer printed on the secondcolorless digital primer) remained substantially intact.

It is to be understood that the ranges provided herein include thestated range and any value or sub-range within the stated range. Forexample, a range from about 30 nm to about 300 nm should be interpretedto include not only the explicitly recited limits of about 30 nm toabout 300 nm, but also to include individual values, such as 75 nm, 150nm, 111.5 nm, 225 nm, etc., and sub-ranges, such as from about 100 nm toabout 250 nm, from about 75 nm to about 200 nm, etc. Furthermore, when“about” is utilized to describe a value, this is meant to encompassminor variations (up to +/−10%) from the stated value.

In describing and claiming the examples disclosed herein, the singularforms “a”, “an”, and “the” include plural referents unless the contextclearly dictates otherwise.

Reference throughout the specification to “one example”, “anotherexample”, “an example”, and so forth, means that a particular element(e.g., feature, structure, and/or characteristic) described inconnection with the example is included in at least one exampledescribed herein, and may or may not be present in other examples. Inaddition, it is to be understood that the described elements for anyexample may be combined in any suitable manner in the various examplesunless the context clearly dictates otherwise.

While several examples have been described in detail, it will beapparent to those skilled in the art that the disclosed examples may bemodified. Therefore, the foregoing description is to be considerednon-limiting.

What is claimed is:
 1. A digital method for priming a substrate forsubsequently receiving liquid electrophotographic (LEP) ink, the digitalmethod comprising: selecting a colorless primer, including: a resinswollen with a non-polar carrier fluid; a solid polar compound dispersedin the swollen resin, the solid polar compound being selected from thegroup consisting of a cellulose microcrystalline powder, dextrin,maltose monohydrate, polyacrylic acid, polyvinyl alcohol, a styrenemaleic anhydride copolymer, a bismaleimide oligomer, sucrose, sucroseoctaacetate, sucrose benzoate, and combinations thereof; a chargeadjuvant; and additional non-polar carrier fluid present in an amountsuch that a total solids content of the colorless primer ranges fromabout 1% (w/w) to about 5% (w/w); and applying the colorless primerdirectly to a surface of the substrate.
 2. The digital method as definedin claim 1 wherein the styrene maleic anhydride copolymer is selectedfrom i) styrene maleic anhydride resin, cumene end-capped and ii) acopolymer of styrene and dimethyl amino propylamine maleimide.
 3. Thedigital method as defined in claim 1, further comprising selecting thesubstrate from the group consisting of corona treated polymeric(bi-oriented polypropylene), a coated paper, and an uncoated paper. 4.The digital method as defined in claim 1 wherein the applying of thecolorless primer is accomplished using liquid electrophotographicprinting.
 5. The digital method as defined in claim 1 wherein theselected colorless primer further includes a charge director.
 6. Thedigital method as defined in claim 1 wherein the selected colorlessprimer includes: an isoparaffinic hydrocarbon as the non-polar carrierfluid; any of an ethylene methacrylic copolymer or an ethylene acryliccopolymer as the resin; the polyacrylic acid as the solid polarcompound; and aluminum stearate as the charge adjuvant.
 7. A colorlessdigital primer, comprising: a colorless paste, including: a non-polarcarrier fluid; and a resin swollen in the non-polar carrier fluid, theresin present in the paste in an amount ranging from about 20% (w/w) toabout 50% (w/w); and a solid polar compound dispersed in the resin, thesolid polar compound being selected from the group consisting of acellulose microcrystalline powder, dextrin, maltose monohydrate,polyacrylic acid, polyvinyl alcohol, a styrene maleic anhydridecopolymer, a bismaleimide oligomer, sucrose, sucrose octaacetate,sucrose benzoate, and combinations thereof, and the solid polar compoundbeing present in an amount up to 60 wt. % of solids in the colorlesspaste.
 8. The colorless digital primer as defined in claim 7 wherein thecolorless digital primer is in a concentrated form having a total solidscontent ranging from about 20% (w/w) to less than 100% (w/w).
 9. Thecolorless digital primer as defined in claim 7, further comprisingadditional non-polar carrier fluid so that a total solids content in thecolorless primer ranges from about 1% (w/w) to about 5% (w/w).
 10. Thecolorless digital primer as defined in claim 7 wherein the styrenemaleic anhydride copolymer is selected from i) styrene maleic anhydrideresin, cumene end-capped and ii) a copolymer of styrene and dimethylamino propylamine maleimide.
 11. A method of making a colorless digitalprimer, comprising: selecting a solid polar compound from the groupconsisting of a cellulose microcrystalline powder, dextrin, maltosemonohydrate, polyacrylic acid, polyvinyl alcohol, a styrene maleicanhydride copolymer, a bismaleimide oligomer, sucrose, sucroseoctaacetate, sucrose benzoate, and combinations thereof; adding thesolid polar compound to a colorless paste including a non-polar carrierfluid and a resin; and grinding the solid polar compound and thecolorless paste to disperse the solid polar compound in the resin of thecolorless paste.
 12. The method as defined in claim 11 wherein selectingthe solid polar compound includes selecting the styrene maleic anhydridecopolymer from i) styrene maleic anhydride resin, cumene end-capped, andii) a copolymer of styrene and dimethyl amino propylamine maleimide. 13.The method as defined in claim 11, further comprising pre-grinding thesolid polar compound to reduce a particle size of the solid polarcompound to be within a range of about 30 nm to about 300 nm.
 14. Themethod as defined in claim 11, further comprising: adding a chargeadjuvant during the grinding; diluting the colorless paste withadditional non-polar carrier fluid; and adding a charge director afterthe grinding and the diluting.
 15. The method as defined in claim 11wherein the grinding is accomplished at a temperature ranging from about25° C. to about 40° C. for a time ranging from about 12 hours to about48 hours.